The invention concerns a process for the manufacture of a spunbonded fabric of thermobonded curled bicomponent fibers, whereby the bicomponent fibers consist of two plastic materials with different properties.
Such a process is disclosed in EP 0 391 260 A1 where composite fibers placed on a screen belt are curled and adhere to each other through the effect of heat, i.e. heated air. In this process, cold air is blown from below through the sieve belt shortly before hot air is supplied to fluff the nonwoven fabric. Such a nonwoven fabric is up to about 2.3 mm thick. Nonwovens created by such processes can be used in hygienic products such as diapers or sanitary napkins as an acquisition layer. For some applications these nonwovens are too thin or have too hard a feel and are also not voluminous enough. A similar process is also disclosed in U.S. Pat. No. 5,382,400.
Common to these and other processes is the fact that the filaments are curled solely by the treatment with heat. Thermobonding also occurs while treating the filaments with heat, i.e. the individual fibers are adhered at their bonding points. The nonwoven fabric obtained in this manner does not satisfy all requirements, especially when used for hygienic products, in particular they are frequently insufficiently voluminous.
The problem of the invention is to present a process of the initially mentioned type that can create spunbonded fabrics from thermobonded curled bicomponent fibers that are voluminous, and that can specifically influence the properties of the nonwoven fabric with regard to strength, porosity, curling of the individual fibers and final weight. If needed, the watertightness of the nonwovens created by the process is to be influenced by the process.
In the invention process to manufacture spunbonded fabrics of thermobonded, curled bicomponent fibers where the bicomponent fibers consist of two plastic materials with different properties, the fibers spun from the two plastic materials are drawn off uncurled and stretched in a first step, placed on a sieve belt and thermobonded in a second step, and the bicomponent fibers of the nonwoven fabric formed in this manner are stretched lengthwise and/or transversely and curled by heat treatment in a third step.
It was surprisingly shown that the properties of the nonwoven fabric could be directly influenced when the nonwoven fabric is stretched lengthwise and transversely during heat treatment. The actual curling of the bicomponent fibers in the invention process is obtained during relaxing following lengthwise and/or transverse stretching. The rate of curling can be increased by the combined thermal and mechanical influences on the fibers in the spunbonded fabric whereby the nonwoven fabric obtained in this way becomes more voluminous than the nonwoven fabrics obtained with prior art processes. With nonwoven fabrics created by conventional processes, part of the curling and hence part of the volume is lost during thermobonding and other heat treatments.
It was found to be advantageous to curl the nonwoven fabric by mechanical stretching only after thermobonding, thereby increasing the volume of the nonwoven fabrics obtained in this way compared with those obtained by conventional processes.
A larger volume is particularly advantageous when the nonwoven fabric made by the process according to the invention is used as an acquisition layer in diapers and sanitary napkins.
In one advantageous embodiment of the process, the latter is carried out with bicomponent fibers of the side-by-side type. In another embodiment of the invention the process is carried out with bicomponent fibers with an eccentric core. By using solid or hollow bicomponent fibers of the side-by-side type and the type with an eccentric core, a softer product feel can be obtained. This soft feel allows all nonwoven fabrics previously used for hygiene to be replaced.
Another embodiment of the invention provides that the process be carried out with segmented hollow bicomponent fibers where neighboring segments of each fiber consist of different materials. In another embodiment the fibers of the nonwoven fabric are stretched and heat-treated such that the fibers curl and fibrillate. In this, use was made of the knowledge that the stretching especially of segmented bicomponent fibers beyond a certain degree of stretching will result in fibrillation, i.e. a controlled tearing open or splitting open occurs. When the fibers stretched and fibrillated in this way are relaxed again, they are curled in addition to fibrillation.
A nonwoven fabric obtained in this manner possesses the same basis weight yet clearly lower porosity which makes the nonwoven fabric more watertight.
In one advantageous embodiment of the invention the fibers are spun from two polypropylene granulates. The MFI viscosity range of the first polypropylene granulate is 16 to 35 and the MFI viscosity range of the second polypropylene granulate is 8 to 10 points below the MFI viscosity range of the first polypropylene granulate.
Alternatively, the fibers can be spun from two different polyolefins, or a polyolefin and PET.